1. Field of the Invention
This invention generally relates to a method and apparatus for examining the static characteristic, such as an inclination angle, and the dynamic characteristic, such as wobble and steering angle, of a wheel of a vehicle, such as an automobile, and, in particular, to a high accuracy wheel examining method and apparatus capable of carrying out static measurements, in which measurements are taken with a wheel in a non-rotating condition, and dynamic measurements, in which measurements are taken with a wheel in a rotating condition.
2. Description of the Prior Art
An apparatus for examining the mounting condition and/or alignment of a wheel of an automobile or the like is well known. A wheel attached to a vehicle, such as an automobile, is set with various conditions, including the so-called inclination angles, such as a toe angle, a camber angle and a caster, as the static characteristics of a wheel. These inclination angles are checked before a vehicle after having been manufactured is shipped and also after a maintenance or repair service operation involving replacement of wheels has been conducted. In order for a vehicle to have an excellent running performance, it is important that the inclination of a wheel be accurately set. In addition, the dynamic characteristic of a wheel, i.e., the characteristic of a wheel in rotation, includes wobbling of a wheel to the left and to the right and the angle of a wheel steered, and the running performance of a vehicle is significantly influenced by such a dynamic characteristic, so that it is important that such a dynamic characteristic can be measured at high accuracy.
A prior art wheel examining apparatus capable of measuring the dynamic characteristic of a wheel of a vehicle by having the wheel supported on a pair of support rollers and clamping both sides of the wheel with clamp rollers is described in the Japanese Patent Laid-open Pub. No. 63-286744, which corresponds to the U.S. Pat. No. 4,901,560, issued Feb. 20, 1990 to Hirano et al., which is hereby incorporated by reference. One of the co-inventors of the invention of this application is also one of the co-inventors of the above-identified U.S. patent. In the wheel examining apparatus disclosed in the above-identified U.S. patent, a support roller assembly 30 including a pair of support rollers is provided to be movable in a plane translationally as well as rotationally, and a rotating object thrust absorbing mechanism is incorporated in the support roller assembly 30. That is, as illustrated in FIG. 1 of the above-identified U.S. patent, in particular its FIGS. 13 and 14, the support roller assembly 30 has a frame 32 which is formed with a first engaging means 32a, and positioning means 34a-34b, which is fixedly provided exteriorly, is provided with a second engaging means 33 which can be engaged with and disengaged from the first engaging means 32a. And, when the first engaging means 32a and the second engaging means 33 are brought into engagement, the support roller assembly 30 pivots around a pivotal point defined by the engagement between the first and second engaging means 32a and 33, thereby absorbing the thrust applied by the wheel 1 in rotation.
As also shown in FIG. 1 of the above-identified U.S. patent, the support roller assembly 30 is rotatably mounted on a floating support table 20, and since the floating support table 20 is provided to be movable translationally in a plane by means of LM (linear motion) guides including rails extending in the longitudinal and transverse directions, the support roller assembly 30 can execute not only a translational motion, but also a rotational motion in a plane. As a result, the support roller assembly 30 can execute any two-dimensional motion in a plane freely. Accordingly, when both sides of a wheel 1 supported on the support rollers 31 are clamped by clamp rollers 47, the center of wheel 1 becomes aligned with the geometrical center of a clamping device including clamp rollers 47. In this case, if the support roller assembly 30 is unstrained in motion, the center of wheel 1 is brought into alignment with the geometrical center of the clamping device accurately irrespective of the fact that wheel 1 is located at the center of support rollers 31 or at any location slightly shifted therefrom.
However, in the apparatus shown in the above-identified U.S. patent, if wheel 1 is located on support rollers 31 at a location relatively away from the center thereof when wheel 1 placed on support roller assembly 30 has been clamped by clamp rollers 47 on both sides, first engaging means 32a may not be in alignment with second engaging means 33 any more. Thus, when second engaging means 33 is moved forwardly to bring it into engagement with first engaging means 32a before setting wheel 1 in rotation, second engaging means 33 may fail to be brought into engagement with first engaging means 32a because of such a positional misalignment. Therefore, in this wheel examining apparatus, while floating support table 20 on which support roller assembly 30 is mounted resides in its initial home location prior to setting the floating support table in an unstrained condition, it is necessary to move second engaging means 33 in the forward direction to have it brought into engagement with first engaging means 32a of support roller assembly 30.
In the wheel examining apparatus described in the above-identified U.S. patent, as shown in FIG. 1 of the present application, when placing wheel 1 to be examined on support roller assembly 30, since floating support table 20 and thus support roller assembly 30 is locked at its predetermined initial location, it is necessary to move second engaging means 33 in the forward direction to be brought into engagement with first engaging means 32a of support roller assembly 30 while floating support table 20 is in its locked condition. Thereafter, wheel 1 is clamped by roller clamps 47 from both sides to thereby effect positioning of the center of wheel 1. In this case, since floating support table 20 and thus support roller assembly 30 is partly restrained in movement due to engagement between first and second engaging means 32a and 33, it cannot be said that floating support table 20 is in a complete floating condition. Therefore, with the above-described structure, positioning of the center of wheel 1 is not entirely satisfactory in terms of reliability and accuracy.
Furthermore, in the wheel examining apparatus disclosed in the above-identified U.S. patent, as shown in FIG. 1 of the present application, wheel 1 is placed on support roller assembly 30 with support roller assembly 30 and thus floating support table 20 locked, and then first and second engaging means 32a and 33 are brought into engagement to establish a thrust absorbing mechanism, thereby setting up a condition in which a thrust absorbing operation can be carried out. Then, support roller assembly 30 and thus floating support table 20 is unlocked and thus set in an unstrained condition. Then, clamp rollers 47 are activated to clamp wheel 1 from both sides. Then, wheel 1 is set in rotation to take measurements which are then processed according to a predetermined program. The resulting toe angle value is displayed and alignment adjustments are effected, if necessary.
In this instance, the toe angle of a wheel is determined such that a longitudinal reference center line of a vehicle is first determined based on the center of each of a plurality (typically four) of wheels of a vehicle which is determined by clamping the wheel with clamp rollers from both sides and the toe angle of each of the wheels is determined as an angle formed between the longitudinal reference center line and the direction or center line of the wheel. And, if the measured toe angle of a wheel differs from a desired value, the alignment of the wheel is adjusted. In this case, however, when the wheel is adjusted in alignment, the location of the center of the wheel shifts, so that the longitudinal reference center line of a vehicle also shifts. That is, as shown in FIGS. 2 and 3, a wheel 1 is typically set with various parameters, including a king pin offset and a caster trail. It is also common that the wheel 1 is set with a predetermined camber. Under the condition, i the wheel is adjusted depending on measured values, the location of the center of a wheel shifts as shown in FIG. 4, so that the new location of the center of a wheel would not be the same as that before adjustments. Such a deviation would cause an error in toe angle measurements. In addition, the movement of a wheel during such adjustments is, in fact, a very complicated three-dimensional movement, so that detector rollers in contact with both sides of the wheel cannot follow the movement of the wheel accurately, which would cause discrepancy in contact point between the wheel and the detector rollers, which, in turn, also contribute to shift the location of the center of the wheel because the location of the center of a wheel is determined as the center location of the clamp rollers clamped on both sides of the wheel. Such a deviation would also cause an error in the measurement of a toe angle.